Electroplating



March 1953 s. s. JOHNSTON ELECTROPLATING Filed Feb. 10, 1955 INVENTORSAMUEL S. JOHNSTON BY aw ATrRNEY United States Patent {3 FELECTRDPLATING Samuel S. Johnston, Steubenvill'e, Ohio, assignor toNational Steel Corporation, a' corporation of Delaware ApplicationFebruary 10, 1953, Serial No. 336,124 3 Claims. (CL 204-43) Thisinvention relates to a method of electroplating strip material with acoating of metaland is primarily concerned with electroplating ferrousmetal strip, such asblack plate, with a coating of protectivemetal, suchas tin, while the strip is moving at a high speed.

When continuously and progressively electroplating strip steel with tin,the strip is continuously moved through a long electroplating line atspeeds of about 1,000 feet per minute to 2000 feet per minute andhigher. The electroplating line includes a large number of unitsdisposed along the path of the strip for treating and electroplating thestrip while traveling at such high speeds. These units include a unitfor cleaning the strip and a unit for scrubbing and washing the cleanedstrip to remove any cleaning solution dragged out of the cleaning unitby thestrip. In addition, the electroplating line includes a unit forpickling the cleaned and. washed strip and a unit for scrubbing andwashing the pickled strip with cold water to remove any residual pickleliquor dragged out of the pickling apparatus. This wet, washed strip isthen passed through the electroplating unit in which at least onesurface of the strip is maintained in contact with a bath ofelectroplating solution. The clec: troplating unit may include one, andusually a plurality, of electroplating cells in each of which there isprovided one or more tin anodes. As the strip moves across or throughthe electroplating cells, current is passed from the anodes through theelectroplating solution to the strip to deposit thereon a layer of anodemetal. The electroplating solution will include at least one electrolyteof the metal which is to be plated. Usually, only a thin film of tin isdeposited by the electroplating process and for the tin plated productto have sufficient resistance to attack it is necessary that the coatingbe of the best quality that can be deposited. Accordingly, it isnecessary that the units of the electroplating line including theelectroplating unit all be operated at peak etficiency in order toproduce satisfactory electro-tinplate. Further, it is diflicult tooperate such a line at peak efliciency so that at times the quality ofthe coating will not be satisfactory for reasons which can not bedete'rmined. As a result, there is a need for an improved electroplatingmethod and apparatus which will produce better coatings and which willnot be as difficult to opcrate at peak efiiciency as has been the caseheretofore.-

The value of a metal coating such as tin on a-base' metal depends in ageneral way on the uniformity of the thickness of coating obtained. Lackof uniformity in the thickness of a metal coating not only adverselyaifects its protective value but also contributes to a product of poorappearance and renders it unsatisfactory for many uses. Although theexact cause of ununiform thickness of coatingv is not known in manyinstances, there are several knowntypes which may be classified byphysical appearance. One such type is known as striation. Striation is astriped or streaked efiect running the length of the strip, having noparticular fixed pattern, but being of difierent width along the2,825,681 Fatented- Mar. 4, 1.958

length of the strip; Striations occur in the center twothirds portion ofthe strip, the outer limits of the width of the strip usually beingfreeof striation. Another ef-" fect often occurring is one which is referredto as anode pattern. Anode pattern is a striped etfect, the stripesbeing parallel, fairly uniform in width and running along the length ofthe strip.

Both striation and anode pattern efiects may occur at the same time withthe result that the product is highly unsatisfactory from the standpointof both, protective value and appearance.

Accordingly, it isanobject of the present invention to provideanim'proved method of electroplating the con; ductivevsurfac of materialwith a metallic coating: which may include one or a plurality. ofmetals.

Another object of the present invention isto provide an improvedelectrodeposited coating of protective metal.

A further objec't of the present invention is to provide an improvedmethod'of electroplating ferrous metal strip with tin.

It is also an. object of. the present invention to provide an improvedmethod of electroplating strip material at a high rate of speed with aprotectivecoating of metal of superior quality.

A further object of the present invention is to provide an improvedmethod of electroplating strip materials at a high rate of speed with aprotective coating of metal of uniform thickness.

Another object ofthe present invention is to provide an improved methodof electroplating strip materials at a high rate of speed with aprotective coating of metal relatively free of striation and anodepattern.

These. and other objects and advantages willbecome more apparentwhenconsidering the following detailed description, taken with theaccompanying drawing, in which: 7

Figure l is a side view, partially in section, diagrammatically showinga portion of a strip plating line em bodying the principles of thepresent invention; and

Figure 2 is a plan view of the apparatus of Figure. 1.

In accordance with the present invention, the quality of anelectroplated tin coating is improved if, before the wet-washed strip iselectroplated, the strip is heated to a temperature of through F. bybringing a heated aqueous solution containing at least one of theelectrolytes and preferably all of the electrolytes of tin present inthe electroplating solution into intimate contact with the surface to beplated.

it is to be understood that the present invention is not limited to anyparticular theory but apparently the very thin aqueous film. which ispresent on the washed strip reduces the efiectiveness of theelectroplating action .in' the electroplating unit and causes striation.It isfln'ot known whether the aqueous film insulates the strip to re;duce the current passing to the strip from the anodes, or whether theaqueous film dilutes the electroplatingsolution adjacent the strip, or'whether the aqueous film reduces the quality of the electrodeposit forsome other reason. Whatever the exact cause is, it has been dis coveredthat the quality of the plated tin can be'improved and striationeliminated by replacing the aqueous film out the washed strip with afilm solution that is more nearly like or is substantially the same asthe electroplating solution. Film replacement is obtained by physicallypropel ling the tin electrolyte containing solution against the's un'faces to be plated. The aqueous rinsingsolution is pref-i erablybstantially the same as the electroplating 5 111: tion in so far asthe'electrolyte or electrolytes of in g concerned. A rinsing solutioncontaining only a' very'. small amount of'tin electrolyte'canbe used butbetter re sultsare obtainedif the rinsing solution contains about thesame amountof the same tin electrolyte present in the' 7 Referring tothe drawings, the strip which usually will have been previously cleanedand washed by apparatus,not shown, progressively passes over therotatably supported; rubber-covered roll. 10 to apickler 11 where thestrip 'is treated in a suitable pickling solution containing one or moreacids; The pickled strip then moves to a scrubbing and washing apparatus12 where the residual pickling solution is washed off by water. Thewashed strip passes from the scrubber 12 through the rinsing unit 13 andthen to the electroplating unit'14 which may be ofany suitable type. Theelectroplating unit 14 may be of the type in which the strip is passedthrough a bath so that both surfaces are simultaneously electroplated.Asis shown in the drawings, electroplating unit '14 is of the type inwhich the strip is passed across a plurality of electroplating cells inwhich the lower surface of the strip is in contact with theelectroplating bath in each cell so that the lower face is plated withtin. If the opposite surface of the strip is to be plated, the directionof travel is reversed and the strip is brought back across. a secondseries of electroplating cellswith the opposite surface facingdownwardly and in contact with the electroplating solution in the cellsso that this opposite surface is' plated withtin. Such anelectroplatingline is shown in the bookentitled Tinplate by Hoare andHedges, l945, .page 263.i After the strip has been electroplated withtin, the plated strip may be treated in any desired manner.

' The pickling apparatus ll includes a tank 16 in which there aremounted a pair of rotatable, rubber-covered rolls 17 for depressing thestrip '8 and causing it to pass through the body of pickling solution 18in the tank 16. Apair of rotatable, rubber-covered wringer rolls 19 areprovided to remove the excess pickle liquor and prevent the strip fromdragging an excessive amount of pickling solution out of the tank 16.When the strip leaves the wringer rolls 19 at 20, it has thereon a filmof pickling solutionand it then passes through the scrubber 12 whichremoves this residual pickling solution. The scrubber includes a firstbrush 21 and a cooperating lower back-up 'roll 22. A'plurality of spraypipes 23, each connected to a source of wash water, are arranged todirect water sprays 24 against the strip as it passes the brush21. Thescrubber 12 includes a second brush 26 and a cooperating upper back-uproll 27. A plurality of pipes '28 are each connected to a source of washwater and are ar-i ranged to direct streams orsprays of water 29againstthe strip as it passes between the brush 26 and the backup roll 27. 'Thebrushes 21 and 26 maybe driven and rotated in'a direction opposite tothe direction of strip travel. Any suitable means may be provided fordriving the brushes; A tank 30 is provided for collecting the excesswash water. A pair of wringer rolls 32 are provided for removing 'excessliquid from the strip.

Guide rolls 33 and 34 are provided adjacent the rinsing unit '13. Therinsing unit 13 includes a tank 35 and a source of supply 36 of rinsingsolution. .The source of supply 36 of rinsing solution is arranged fordirecting jetsor streams pr rinsing solution'37against both sides of the'central portion of the strip S as it entersthe tank 35. Rinsingsolution 45 is preferably fed to supply means 36 from a recirculating.tank 38.; This is done by means 0t pipeline 3? :a'rid'pump 4(LfTherinsing solution from V to one side of the generator 53.

pump 40 passes through a heat exchanger 41 to the sup ply means 36. Heatexchanger 41 will be described more fully hereinafter. An outlet pipe42'is provided for tank 35 and preferably is valve controlled so asto'maintain the body of solution in tank 35 at the proper level. Excessrinsing solution is removed from strip S after passing through therinsing unit 13 by contact roll 55 and back-up roll 57. V I

Electroplating unit 14 includes a plurality of similar electroplatingcells 43, only one of whichwill be de scribed in detail. Theelectroplating cell 43 includes a tray 44 having side troughs 45 and endtroughs 46 for collecting the overflowing electroplating solution.Electroplating solution is continuously supplied to the tray from asource 33 of electroplating solution by means described hereinafter. Thesides of the tray 44 are of such a height as to maintain a body or bathof electroplating solutionin the tray. with the upper surface at orslightly above the level of the lower surface of strips. Electroplatingsolution is constantly fed to the tray to replenish the bath and thesolution continuouslyflows over the sides of the tray 44 into thecollecting troughs 45 and 46. In the tray there is a center anodesupport 48 and spaced side supports 49 on which are mounted a pluralityof anodesSil, each formed of tin. The center support is provided withelectrically conductive surfaces which engage the ends of the anodes 5i)and these surfaces are connected through a bus bar 51 and a wire-52 Suchanarrangement of anodes is shownand described in'Patent No. 2,399,254,issued April 30, 1946 to Rieger 'et al. Below each end of the'tray 44there is a collecting pan 54 and these pans 54 collect theelectroplating solution-dragged out of the electroplating trays by thestrip andalso collect the solution from the overflow troughs. At theentrance side of tray 44, there is a conductive contact roll 55connected through wire 56 to'. the other sideof generator 53. Arotatable back-up roll'57 is provided forholding the strip in contactwiththe contact roll 55. Theisides of the tray 44 are so arranged thatthe stripis in contact with the upper surfacefof the body'ofelectroplating'solution in the tray so that'electrical current flowsfrom the anodes through the electroplating solution to .the striptodeposit a layer of tin .on the bottom surface of the strip. r i

The electroplating solution may be of any particular type, for example,the stannous sulphate acid type or any other suitable type whichcontains oneor more tin electrolytes (see United StatesPa.tents.2,512,7l9 and 2,598,- 486). contains stannous sulphate which isanelectrolyte ofthe metal being plated. As the tin ions migrate to andare deposited on the strip which'is the cathode, the bath is replenishedwith tin dissolved from the anodes. N01? mally, the electroplatingsolution flows. from the collecting trays 54 through drain pipes 5 9 toa common collecta ing or recirculating tank 38 which isjreplenished whennecessary with chemicalsto maintain the electroplating solution in theproper condition. Tank 38 preferably is located below and extends thecombined length of Washingunit 13 and the electroplating units 14. Heatexchanger coils 69 are provided for controlling the temperature of theelectroplating solution in tank 38. The temperature of thiselectroplating solution will vary in ac-' cordance with such conditionsas length of time that the line has been in operation since the lastshut down,'current density being employed for electroplating, ambienttemperature,- temperatureof make-up solution added to the electroplatingsolution, 1 etc. Consequently, heat ex,-

changer coils 59 may be used either for heating or cooling theelectroplating solution as required. Lines 61, pumps 62, manifold '63and jet'or spray orifices- 64 are provided 7 for supplyingelectroplating solution'tothe individual cells 14 at the forward endthereof and physically pro-.3

The stannous sulphate acid bathgfor example, 7

pelllng the same against the central area of both surfaces of the strip.

The rinsing solution 37 contains an electrolyte of tin which is the sameas at least one of the tin electrolytes in the electroplating solution.The character of the final coating is improved when the rinsing solutioncontains tin electrolyte and apparently the reason is that the aqueousfilm on the strip is replaced with a him of rinsing solution which ismore nearly like the electroplating solution. The greatest degree ofimprovement is obtained when the rinsing solution is the same as theelectroplating solution in the electroplating unit 14-. Consequently,the preferred embodiment of the present invention utilizeselectroplating solution from tank 38 and receives it through line 36 vialine 59, pump it! and heat exchanger 41. If it were desirable to providerinsing solution containing at least one tin electrolyte from anothersource, this could be accomplished by directing the line 39 to thatsource and providing for the return of overflow from tank 35 throughline 32 to that same source. Heat exchanger 41 provides the heatnecessary for the rinsing solution to obtain a strip temperature of 120through 160 F. Steam or hot water may be used for this purpose.

If the strip is electroplated when it is at a temperature below 120 F.,anode pattern effects result. If, on the other hand, the strip is heatedto a temperature above 160 F., decomposition of certain components inthe electroplating solution, such as ferro-cyanide compounds will occur.These factors therefore determine the upper and lower temperature rangesof the strip at the time of commencing the electroplating operation.Although the pickling solution 18 is maintained at a temperature ofapproximately 180 F the rinse water in tank 12 is at the prevailingwater supply temperature because of the fact that cold rinse waterremoves the pickling acid better and prevents the strip from drying.Therefore, upon emergence from roll 32, the strip is at a temperaturebelow 120 F. Thus, in order to obtain the benefits of the presentinvention, it is necessary to heat the strip prior to electroplating.

Although the preferred temperature of the strip upon commencingelectroplating is somewhat dependent upon the coating weight of the tinto be applied, it has been found that a more important factor is thecurrent density being used. For example, when using a current density of200 amperes-per square foot, it has been found that the temperature ofthe strip should be at least 125 F. for preferred results. Similarly, atcurrent densities of 300 and 600 amperes per square foot, the minimumtemperature should be 135 and 145 F., respectively. In no case shouldthe strip temperature exceed 160 F.

The present invention has been described in connection with theelectroplating of tin on ferrous metal strip, but may be used forplating other metals onto other strip materials. Although the rinsingsolution preferably is substantially the same as the electroplatingsolution, it need not be the same and it does constitute an inprovementwhen the rinsing solution contains only a small amount of the sameelectrolyte as the electroplating solution.

I claim:

1. The method of eliminating striation and anode pattern in theprogressive electrotinplating of a surface of ferrous metal strip atstrip speeds of about 1000 feet per minute and higher, comprising thesteps of pro gressively passing the strip at a speed of at least about1000 feet per minute through a washing zone, a rinsing zone and anelectrotinplating zone, washing the surface of the strip to be platedwith relatively cold water in the washing zone, passing a stream of hotaqueous rinsing solution into the rinsing zone, heating the strip in therinsing zone in the absence of electrolytic treatment of the strip to atemperature within the range Fahrenheit to Fahrenheit by continuouslybringing the hot aqueous rinsing solution into intimate contact with thesurface to be plated, and electrotinplating the surface of the heatedstrip in the electroplating zone while the surface to be plated is incontact with a bath of aqueous electrotinplating solution, theelectrotinplating solucontaining at least one tin electrolyte and therinsing solution containing said one tin electrolyte.

2. The method of eliminating striation and anode pattern in theprogressive electrotinplating of a surface of ferrous metal strip atstrip speeds of about 1000 feet per minute and higher, comprising thesteps of progressively passing the strip at a speed of at least about1000 feet per minute through a washing zone, a rinsing zone and anelectrotinplating zone, washing the surface of the strip to be platedwith relatively cold water in the washing zone, passing a stream of hotaqueous rinsing solution into the rinsing zone, heating the strip in therinsing zone in the absence of electrolytic treatment of the strip to atemperature within the rang 120 Fahrenheit to 160 Fahrenheit bycontinuously bringing the hot aqueous rinsing solution into intimatecontact with the surface to be plated, and eleetrotinplating the surfaceof the heated strip in the electroplating zone while the surface to beplated is in contact with a bath of aqueous electrotinplating solution,the electrotinplating solution containing at least one tin electrolyteand the rinsing solution containing substantially the same percentage ofsaid tin electrolyte as the electrotinplating solution.

3. The method of eliminating striation and anode pattern in theprogressive electrotinplating of a surface of ferrous metal strip atstrip speeds of about 1000 feet per minute and higher, comprising thesteps of progressively passing the strip at a speed of at least about1000 feet per minute through a washing zone, a rinsing zone and anelectroplating zone, washing the surface of the strip to be plated withrelatively cold water in the washing zone, passing a stream of hotaqueousrinsing solution into the rinsing zone, heating the strip in therinsing zone in the absence of electrolytic treatment of the strip to atemperature within the range 120 Fahrenheit to 160 Fahrenheit bycontinuously bringing the hot aqueous rinsing solution into intimatecontact with the surface to be plated, and electrotinplating the surfaceof the heated strip in the electroplating zone while the surface to beplated is in contact with a bath of aqueous electrotinplating solution,the electrotinplating solution containing at least one tin electrolyteand the rinsing solution and the electrotinplating solution beingsubstantially the same.

References Cited in the file of this patent UNITED STATES PATENTS2,497,894 Luke Feb. 21, 1950 2,513,859 Glock July 4, 1950 2,557,823Holbrook June 19, 1951 2,563,229 Faust et al Aug. 7, 1951 2,569,577Reading Oct. 2, 1951 2,689,399 Gray Sept. 21, 1954 OTHER REFERENCESMetal Finishing, February 1944, pages 77-79. The Iron Age, April 30,1942, pages 35-44.

U. S DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTIONPatent N00 2,825,681 March 4, 1958,

Samuel S, Johnston It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionand that the said Letters Patent should read as corrected below.

Column 2, line 43, for "Wet==Washed" read Wet, washed --5 column 6, llne24, for "rang" read range Signed and sealed this 29th day of April 1958a(SEAL) Attest:

KARL Ha MINE ROBERT C. WATSON Attesting Officer Commissioner of Patents

1. THE METHOD OF ELIMINATING STRIATION AND ANODE PATTERN IN THEPROGRESSIVE ELECTROTINPLATING OF A SURFACE OF FERROUS METAL STRIP ATSTRIP SPEEDS OF ABOUT 1000 FEET PER MINUTE AND HIGHER, COMPRISING THESTEPS OF PROGRESSIVELY PASSING THE STRIP AT A SPEED OF AT LEAST ABOUT1000 FEET PER MINUTE THROUGH A WASHING ZONE, A RINSING ZONE AND ANELECTROTINPLATING ZONE, WASHING THE SURFACE OF THE STRIP TO BE PLATEDWITH RELATIVELY COLD WATER IN THE WASHING ZONE, PASSING A STREAM OF HOTAQUEOUS RINSING SOLUTION INTO THE RINSING ZONE, HEATING THE STRIP IN THERINSING ZONE IN THE ABSENCE OF ELECTROLYTIC TREAT MENT OF THE STRIP TO ATEMPERATURE WITHIN THE RANGE 120* FAHRENHEIT TO 160* FAHRENHEIT BYCONTINUOUSLY BRINGING THE HOT AQUEOUS RINSING SOLUTION INTO INTIMATECONTACT WITH THE SURFACE TO BE PLATED, AND ELECTROTINPLATING THE SURFACEOF THE HEATED STRIP IN THE ELECTROPLATING ZONE WHILE THE SURFACE TO BEPLATED IS IN CONTACT WITH A BATH OF AQUEOUS ELECTROTINPLATING SOLUTION,THE ELECTROTINPLATING SOLUTION CONTAINING AT LEAST ONE TIN ELECTROLYTEAND THE RINSING SOLUTION CONTAINING SAID ONE TIN ELECTROLYTE.